Page:Popular Science Monthly Volume 90.djvu/171

With the senders and receivers limited
in their activities to communication be-
tween a single pair of stations, it is usually
not necessary to provide for variation in
the tuned wavelengths of either. That is

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��TELEPHONE

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DETECTOR \

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A simple arrangement which has the detector
in series between the antenna and ground

why the simple arrangement of Fig. 2 In
the December article could be used. When
it is desired to receive from a large number
of outside transmitters, all using different
wavelengths, it is necessary to provide
apparatus whereby the effective length of
the aerial at the receiver may be varied to
suit the incoming wavelength. A large
number of arrangements may be used for
this purpose. Some of them tune the
receiver very sharply, or in other words
make it respond energetically to a very
closely restricted range of wavelengths
for each adjustment. Other sets of connec-
tions are less critical in adjustment, but
easier to handle.

The simplest variable tuning instrument
for use at the receiver is the so-called
"single-slide tuner." This is merely an
inductance coil with a sliding contact
whereby the number of turns in circuit may
be varied at will. It may be used in place
of the tuning coil shown in Fig. 3, and will
allow some latitude of adjustment, though
the tuning is very broad and unsatisfactory.
A better mode of connection for the single-
slide tuner is that of Fig. 4, in which the
detector is put in a side or by-pass circuit;
this gives sharper tuning and fairly strong
signals.

A still better tuning arrangement fuses
the "double-slide tuner," which has two
variable contacts. In the catalogs of
manufacturers of radio apparatus there are
to be found a large number of diagrams

��showing different ways to connect the
double-slide tuner; but the best possible
results are to be secured from the arrange-
ment of Fig. 5. One end of the coil is
connected to ground, and one of the sliders
to the antenna. The larger the amount of
coil between the grounded end and this
first slider, the longer the effective length
of the aerial and the greater the wavelength
for which it is tuned. The grounded end is
also connected to one side of the blocking
condenser described in the November
article, and the other slider is connected
with one terminal of the cr>-stal-detector
also illustrated in November. The tele-
phone has one lead connected with ground,
and the other joins the open sides of the
detector and blocking-condenser. The
test-buzzer, which is not shown in Fig. 5, is
to be arranged as explained in the December
article, so that the cr>stal-detector can be
adjusted to its maximum sensitiveness
without waiting for signals from outside
stations.

Operating the Variable Receiver

In working the apparatus set up as in
Fig. 5, the first step is to make sure that the
detector is adjusted to a sensitive point,
and that the connections are all secure and
in good condition. Then the slider connect-
ed to the detector is set at a position about
half-way along the coil, and the antenna-
slider is moved back and forth slowly along
the length of the entire tuner. When a
station within range starts to send, his

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BL0CKIN6-
CONDtKSER

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��Connecting the detector around the loading-
coil produces a highly selective arrangement

signals will be heard in the telephones; it
will be noted that the dots and dashes are
loudest with the antenna-slider at some
particular setting. Leaving the antenna-
slider at this point, the detector-slider is
moved back and forth until the position
giving the best signals is found. This is the
tuned or approximately tuned adjustment
of the receiver for the specific wavelength
being received.